Harold Leverenz

Clogging in intermittently dosed sand filters used for wastewater treatment

Clogging in intermittent sand filter (ISF) systems was analyzed using an unsaturated flow model coupled with a reactive transport model. Based on the results of a model sensitivity analysis, several variables were determined to be important in the clogging phenomena observed in ISFs, including hydraulic loading rate, influent chemical oxygen demand (COD) concentration, filter dosing frequency, and time of operation. Several modes of operation were identified that minimize the growth of bacteria at the filter surface. Following the sensitivity analysis, several case studies where ISF clogging was documented were simulated using the model. The results from the case study model simulations were found to be correlated with the total suspended solids loading rate (TSSLR) at the point of clogging. A model was developed that relates biomass development at the surface of ISFs with the TSSLR that can be sustained without clogging. The engineering significance of the model is presented in terms of operational and design considerations.

Treatment of Wastewater With Slow Rate Systems: A Review of Treatment Processes and Plant Functions

Land treatment systems constitute a viable alternative solution for wastewater management in cases where the construction of conventional (mechanical) wastewater treatment plants (WWTPs) are not affordable or other disposal options are not available. They have proven to be an ideal technology for small rural communities, clusters of homes, and small industrial units due to low energy demands and low operation and maintenance costs. In addition, slow rate systems (SRS) may be designed using the “zero discharge” concept. The purpose of this article is to review the current trends and developments in the field of SRS, focusing on those systems in which effluent application is based on plant water requirements. Vegetation has an important role in treatment efficiency through its effects on hydraulic loading rate, nutrient removal, and biomass production. In addition, vegetation may affect the fate of trace elements and the degradation/detoxification of recalcitrant organics. Detailed knowledge of the basic processes involved in wastewater treatment and the factors governing the performance of SRS is fundamental for enhancing treatment efficiency and eliminating potential environmental and health risks. Finally, monitoring performance of SRS and adopting the appropriate management strategies are of paramount importance to maintain treatment efficiency over the a long term.

Methane, carbon dioxide, and nitrous oxide emissions from septic tank systems.

Emissions of CH4, CO2, and N2O from conventional septic tank systems are known to occur, but there is a dearth of information as to the extent. Mass emission rates of CH4, CO2, and N2O, as measured with a modified flux chamber approach in eight septic tank systems, were determined to be 11, 33.3, and 0.005 g capita(-1) day(-1), respectively, in this research. Existing greenhouse gas (GHG) emission models based on BOD (biochemical oxygen demand) loading have estimated methane emissions to be as high as 27.1 g CH4 capita(-1) day(-1), more than twice the value measured in our study, and concluded that septic tanks are potentially significant sources of GHGs due to the large number of systems currently in use. Based on the measured CH4 emission value, a revised CH4 conversion factor of 0.22 (compared to 0.5) for use in the emissions models is suggested. Emission rates of CH4, CO2, and N2O were also determined from measurements of gas concentrations and flow rates in the septic vent system and were found to be 10.7, 335, and 0.2 g capita(-1)day(-1), respectively. The excellent agreement in the CH4 emission rates between the flux chamber and the vent values indicates the dominant CH4 source is the septic tank.

Net-zero water management: achieving energy-positive municipal water supply

Net-zero water (NZW) is a new vision for municipal water management, in which significant water is neither imported to, nor exported from the service area, i.e. local water independence. While such a system has long been possible in areas of sufficient water supply and/or sparse population, it is now becoming possible and economical for municipal systems in virtually any modern watershed, through the use of emerging direct potable reuse (DPR) technology. In fact, current implementations are producing design and operating data. Moreover, distributed NZW systems recycling at a high rate are projected to be capable of energy-positive operation, saving more domestic hot water energy than is consumed in treatment. However, NZW and DPR approaches vary widely in terms of source water, source segregation, treatment, and recycling rate. In this study, a workshop was convened to assemble and synthesize a broad cross-section of current NZW and DPR experience, to develop recommendations for water management planning. It was concluded that technology is currently emerging to support widespread NZW management. Recommendations included the introduction of NZW systems into new construction, to be supported by controlled demonstration projects over periods of two years or more; development of supporting regulatory structure with public engagement; development of real-time water quality monitoring devices; and retention of the term “net-zero water” to signify a new water management vision to advance water and energy autonomy.

Evaluation of Greenhouse Gas Emissions from Septic Systems

The emission rates of greenhouse gases (GHGs) from individual onsite septic systems used for the management of domestic wastewater were determined in this study. A static flux chamber method was used to determine the emission rates of methane, carbon dioxide, and nitrous oxide gases from eight septic tanks and two soil dispersal systems. A technique developed for the measurement of gas flow and concentration at clean-out ports was used to determine the mass flow of gases moving through the household drainage and vent system. There was general agreement in the methane emission rates for the flux chamber and vent system methods. Several sources of variability in the emission rates were also identified. The septic tank was the primary source of methane, whereas the soil dispersal system was the principal source of carbon dioxide and nitrous oxide emissions. Methane concentrations from the soil dispersal system were found to be near ambient concentrations, similarly negligible amounts of nitrous oxide were found in the septic tank. All emissions originating in the soil dispersal system were discharged through the building vent as a result of natural, wind-induced flow. The gaseous emission rate data were determined to be geometrically distributed. The geometric mean and standard deviation (sg) of the total atmospheric emission rates for methane, carbon dioxide, and nitrous oxide based on samples from the vent system were estimated to be 10.7 (sg = 1.65), 335 (sg = 2.13), and 0.20 (sg = 3.62) g/capita·d, respectively. The corresponding total anthropogenic CO2 equivalence (CO2e) of the GHG emissions to the atmosphere, is about 0.1 tonne CO2e/capita·yr. This title belongs to WERF Research Report Series . ISBN: 9781843396161 (Print) ISBN: 9781780403359 (eBook)

Development of Design Criteria for Denitrifying Treatment Wetlands

Subsurface wetlands are well suited for on-site applications because they provide odor and vector control and they mitigate public access issues (U.S. EPA, 1993). Artificial subsurface wetlands are typically designed with an inert rock medium, can be either planted or unplanted, and are designed so that the water flows below the surface of the wetlands through the porous medium. The medium provides a surface area for the growth of bacterial films but inhibits the carbon cycling from plant debris because the packing material prevents the plant debris from reaching the water. As a result, subsurface wetlands are only marginally successful at removing nitrogen from wastewater. The nitrogen removal that does occur is the result of plant assimilation and microbial denitrification that utilizes any remaining carbon source in the influent and from plant decay (Kadlec and Knight, 1996). To increase the denitrification performance, an alternative carbon source is required. Gersberg et al. (1983) demonstrated that the addition of carbon, in the form of methanol, stimulated bacterial denitrification and increased nitrate removal efficiencies to 95%. Based on previous research, it has been found that a variety of organic solids can be used simultaneously as media and as a carbon source to support the denitrification process. These include plant biomass (Gersberg et al., 1983), cotton burr and mulch compost (Su and Puls, 2007), wheat straw (Aslan and Turkman, 2003), sawdust (Robertson and Cherry, 1995; Schipper et al., 1998), and woodchips (Healy et al., 2005; Robertson et al., 2009). Schipper et al. (1998) demonstrated that porous groundwater treatment walls amended with sawdust were successful in removing nitrate from contaminated groundwater. Robertson et al. (2005) demonstrated that the proprietary Nitrex filters, which utilize a nitrate reactive material, produced septic tank effluent nitrate removal rates of up to 96%, remaining effective for at least five years, but removal rates were diminished during the winter months. The temperature of the water in a wetland system can significantly affect the rate of denitrification (Kadlec and Knight, 1996). The use of a readily available organic medium in a constructed subsurface wetland as a method for denitrification of nitrified septic tank effluent has not been investigated. This title belongs to WERF Research Report Series . ISBN: 9781843395331 (eBook)